[PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[Alan Maguire]

Reproducible builds [1] require that the same source code with
the same set of tools can build identical objects each time,
but pahole in parallel mode was non-deterministic in
BTF generation prior to

dba7b5e ("pahole: Encode BTF serially in a reproducible build")

This was a problem since said BTF is baked into kernels and modules in
.BTF sections, so parallel pahole was causing non-reproducible binary
generation.  Now with the above commit we have support for parallel
reproducible BTF generation in pahole.

KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
is set, add reproducible_build to --btf_features.

[1] Documentation/kbuild/reproducible-builds.rst

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 scripts/Makefile.btf | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index b75f09f3f424..40bb72662967 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -21,6 +21,10 @@ else
 # Switch to using --btf_features for v1.26 and later.
 pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
 
+ifneq ($(KBUILD_BUILD_TIMESTAMP),)
+pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=reproducible_build
+endif
+
 ifneq ($(KBUILD_EXTMOD),)
 module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
 endif
-- 
2.31.1

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[Jiri Olsa]

On Mon, Jul 01, 2024 at 06:31:33PM +0100, Alan Maguire wrote:
> Reproducible builds [1] require that the same source code with
> the same set of tools can build identical objects each time,
> but pahole in parallel mode was non-deterministic in
> BTF generation prior to
> 
> dba7b5e ("pahole: Encode BTF serially in a reproducible build")
> 
> This was a problem since said BTF is baked into kernels and modules in
> .BTF sections, so parallel pahole was causing non-reproducible binary
> generation.  Now with the above commit we have support for parallel
> reproducible BTF generation in pahole.
> 
> KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
> is set, add reproducible_build to --btf_features.
> 
> [1] Documentation/kbuild/reproducible-builds.rst
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

makes sense

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> ---
>  scripts/Makefile.btf | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index b75f09f3f424..40bb72662967 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -21,6 +21,10 @@ else
>  # Switch to using --btf_features for v1.26 and later.
>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
>  
> +ifneq ($(KBUILD_BUILD_TIMESTAMP),)
> +pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=reproducible_build
> +endif
> +
>  ifneq ($(KBUILD_EXTMOD),)
>  module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
>  endif
> -- 
> 2.31.1
> 

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[Masahiro Yamada]

On Tue, Jul 2, 2024 at 2:32 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> Reproducible builds [1] require that the same source code with
> the same set of tools can build identical objects each time,
> but pahole in parallel mode was non-deterministic in
> BTF generation prior to
>
> dba7b5e ("pahole: Encode BTF serially in a reproducible build")
>
> This was a problem since said BTF is baked into kernels and modules in
> .BTF sections, so parallel pahole was causing non-reproducible binary
> generation.  Now with the above commit we have support for parallel
> reproducible BTF generation in pahole.
>
> KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
> is set, add reproducible_build to --btf_features.
>
> [1] Documentation/kbuild/reproducible-builds.rst
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>




Does not make sense.



KBUILD_BUILD_TIMESTAMP is not a switch for
"please enable the reproducible build".


KBUILD_BUILD_TIMESTAMP requires the build code
to use the given time in the output where timestamps are used.

Your patch does not use the timestamp at all.


If --btf_features=reproducible_build has no downside,
please add it whenever supported.







> ---
>  scripts/Makefile.btf | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index b75f09f3f424..40bb72662967 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -21,6 +21,10 @@ else
>  # Switch to using --btf_features for v1.26 and later.
>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
>
> +ifneq ($(KBUILD_BUILD_TIMESTAMP),)
> +pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=reproducible_build
> +endif
> +
>  ifneq ($(KBUILD_EXTMOD),)
>  module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
>  endif
> --
> 2.31.1
>


-- 
Best Regards
Masahiro Yamada

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[asmadeus]

Masahiro Yamada wrote on Tue, Jul 02, 2024 at 04:58:50PM +0900:
> If --btf_features=reproducible_build has no downside,
> please add it whenever supported.

It makes the build slightly slower (from [1], 3.858 -> 3.991 (+3%) on
my crippled machine for the vmlinux BTF phase -- the modules also get
similar treatment, I'm not sure how big the total time exactly is --
for large kernels with tons of modules it definitely adds up but for small
kernels it's probably "short enough")

[1] https://lkml.kernel.org/r/20240626032253.3406460-1-asmadeus@codewreck.org


I don't particularly mind either way, so this is mostly out of curiosity:
do we have any other setting that would be closer better than this
KBUILD_BUILD_TIMESTAMP to say "make this reproducible", or is the kernel
build supposed to be reproducible except for the date by default?


Thank you,
-- 
Dominique Martinet | Asmadeus

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[Alan Maguire]

On 02/07/2024 08:58, Masahiro Yamada wrote:
> On Tue, Jul 2, 2024 at 2:32 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> Reproducible builds [1] require that the same source code with
>> the same set of tools can build identical objects each time,
>> but pahole in parallel mode was non-deterministic in
>> BTF generation prior to
>>
>> dba7b5e ("pahole: Encode BTF serially in a reproducible build")
>>
>> This was a problem since said BTF is baked into kernels and modules in
>> .BTF sections, so parallel pahole was causing non-reproducible binary
>> generation.  Now with the above commit we have support for parallel
>> reproducible BTF generation in pahole.
>>
>> KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
>> is set, add reproducible_build to --btf_features.
>>
>> [1] Documentation/kbuild/reproducible-builds.rst
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> 
> 
> 
> 
> Does not make sense.
> 
> 
> 
> KBUILD_BUILD_TIMESTAMP is not a switch for
> "please enable the reproducible build".
> 
> 
> KBUILD_BUILD_TIMESTAMP requires the build code
> to use the given time in the output where timestamps are used.
> 
> Your patch does not use the timestamp at all.
>

No, and that's not the intention. It is used as a signal to pahole to
enable reproducibility in parallel build. There is a cost to this so
it's not advisable in all scenarios [1]. Is there a kbuild-approved way
to determine if reproducible builds are in operation?

Alan

[1]
https://lore.kernel.org/dwarves/20240412211604.789632-12-acme@kernel.org/
> 
> If --btf_features=reproducible_build has no downside,
> please add it whenever supported.
> 
> 
> 
> 
> 
> 
> 
>> ---
>>  scripts/Makefile.btf | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
>> index b75f09f3f424..40bb72662967 100644
>> --- a/scripts/Makefile.btf
>> +++ b/scripts/Makefile.btf
>> @@ -21,6 +21,10 @@ else
>>  # Switch to using --btf_features for v1.26 and later.
>>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
>>
>> +ifneq ($(KBUILD_BUILD_TIMESTAMP),)
>> +pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=reproducible_build
>> +endif
>> +
>>  ifneq ($(KBUILD_EXTMOD),)
>>  module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
>>  endif
>> --
>> 2.31.1
>>
> 
> 

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[Masahiro Yamada]

On Tue, Jul 2, 2024 at 6:49 PM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On 02/07/2024 08:58, Masahiro Yamada wrote:
> > On Tue, Jul 2, 2024 at 2:32 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> >>
> >> Reproducible builds [1] require that the same source code with
> >> the same set of tools can build identical objects each time,
> >> but pahole in parallel mode was non-deterministic in
> >> BTF generation prior to
> >>
> >> dba7b5e ("pahole: Encode BTF serially in a reproducible build")
> >>
> >> This was a problem since said BTF is baked into kernels and modules in
> >> .BTF sections, so parallel pahole was causing non-reproducible binary
> >> generation.  Now with the above commit we have support for parallel
> >> reproducible BTF generation in pahole.
> >>
> >> KBUILD_BUILD_TIMESTAMP is set for reproducible builds, so if it
> >> is set, add reproducible_build to --btf_features.
> >>
> >> [1] Documentation/kbuild/reproducible-builds.rst
> >>
> >> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> >
> >
> >
> >
> > Does not make sense.
> >
> >
> >
> > KBUILD_BUILD_TIMESTAMP is not a switch for
> > "please enable the reproducible build".
> >
> >
> > KBUILD_BUILD_TIMESTAMP requires the build code
> > to use the given time in the output where timestamps are used.
> >
> > Your patch does not use the timestamp at all.
> >
>
> No, and that's not the intention. It is used as a signal to pahole to
> enable reproducibility in parallel build. There is a cost to this so
> it's not advisable in all scenarios [1]. Is there a kbuild-approved way
> to determine if reproducible builds are in operation?
>
> Alan
>
> [1]
> https://lore.kernel.org/dwarves/20240412211604.789632-12-acme@kernel.org/




Kbuild already provides a standard mechanism.





config DEBUG_INFO_BTF_REPRODUCIBLE
        bool "Generate reproducible BTF"
        depends on DEBUG_INFO_BTF
        depends on PAHOLE_VERSION >= 126



pahole-flags-$(CONFIG_DEBUG_INFO_BTF_REPRODUCIBLE) =
--btf_features=reproducible_build






Presumably, it is better to add a new section
in Documentation/kbuild/reproducible-builds.rst







--
Best Regards
Masahiro Yamada

Re: [PATCH bpf-next] kbuild, bpf: reproducible BTF from pahole when KBUILD_BUILD_TIMESTAMP set

[asmadeus]

Masahiro Yamada wrote on Mon, Jul 15, 2024 at 08:19:53PM +0900:
> Kbuild already provides a standard mechanism.
> 
> 
> 
> 
> 
> config DEBUG_INFO_BTF_REPRODUCIBLE

Ah, I thought this was already an existing option but it's a suggestion
for a new one.

With my distro hat I have to say I have no interest in chasing more
reproducible knobs for the kernel -- the current trend in 99% of the
projects is that just setting SOURCE_DATE_EPOCH "just works" and the
every non-default option that is required is work for people to track,
enable in various packaging frameworks, backport to old kernels, update
if it ever changes...

I could understand a new option "toggle everything reproducible"
that would select this, even if it doesn't do anything else right now
(perhaps also complain if KBUILD_BUILD_TIMESTAMP isn't set?), but I
can't say I'm thrilled about a new option pahole-specific that would
have to be individually enabled.

(I guess that means I've just volunteered to add both... perhaps next
year when I find time, for now the distro pahole patch toggling
reproducible if SOURCE_DATE_EPOCH is set will do)
-- 
Dominique Martinet | Asmadeus